Air-jet mill and associated pregrinding apparatus for comminuating solid materials

ABSTRACT

An air-jet mill provided with pregrinding chambers and inner sizer suitable for fine grinding (below 10 μ), for cryogenic grinding and also for surface treatment during grinding of hard, elastic and/or thermoplastic materials. 
     During operation there are no moving components to be exposed to heavy wear, and an inner sizer, forming an integral part of the grinding space, uses the energy left after grinding in the system to enable a sharp sizing or separation of fine fractions.

FIELD BACKGROUND OF THE INVENTION

The object of the invention is an energy-saving internal sizing air-jetmill having a pregrinding chamber for fine grinding of preferablyvarious carbides, silicates, oxides, ores, pigments or elasticmaterials, as well as for surface treatment and/or cryogenic grinding ofthe same.

The air-jet mills known according to the present state of art, could bedivided into five basic types. The first type is characteristic of thesituation where grinding comes about by accelerating the material tohigh speed through a nozzle and impacting it against a so-called anvil.This apparatus provides adequate grinding; however, due to high specificenergy consumption its operation is not economical and the liningexperiences much wear thereby causing high contamination of the groundproduct.

In order to eliminate such contaminating effect, it is often thepractice to use linings from the same basic material as the materialwhich is to be ground. The best known version of this type now in use isthe air-jet mill of the Vortex system provided with an outer sizer orseparator, ceramic lining and anvil. The other type of the air-jet millwidely used is the so-called Majec mill. Here, comminution takes placeby the autogenous grinding effect of grains impacting against each otherby the acceleration generated through two nozzles facing one another.This operation, however, exhibits energy losses and thus very poorcomminution efficiency. The nozzles can carry comparatively smallamounts of grains and vortices occur also due to the effect of theopposite air jets thus fairly reducing the number of collision ofgrains. Known examples of this type are: West German Patents DE No.2543691 C2 and DE No. 2523471 C2.

The third air-jet mill, the so called Micronizer type is the one whichhas been used most predominantly. The essence of its operation is thatgrinding takes place in the discusshaped grinding chamber under theeffect of pressurized gas issuing from peripheral jet pipes. The gasjets first contact a circle in the outer third part or half of thegrinding area. Material to be ground enters the grinding space in avertical plane crossing the tangent of this circle, however, at an angleof 60° to the vertical passing through the top of the grinding space.

According to the theory of the designers, the grains greater in sizethan a predetermined dimension are circulating along this tangentialcircle, the smaller grains, that is, the ground end product dischargefrom the facility past obstructing dam entrained in the exhaust and, thecoarser grains, under the effect of the pressurized gas from theperipheral nozzles, collide with each other and circulates until theirdimensions are reduced to or below the required level. Under actualworking conditions the operation of the facility does not meet theconditions of the above theoretical operation, yet the type is widelyused as the unit presenting the best efficiency. Several patentedinventions exist on the above apparatus, e.g. U.S. Pat. No. 3,726,484,SF-33960, DE No. 3201778 C1. These technical solutions represent thecombination of the double-jet mill, the anvil type and the micronizer,where the coarse product is returned to the grinding space, or byapplying a anvil-type pregrinder an attempt is made, with littlesuccess, to improve the fineness of grinding. Therefore, up to now, theunchanged basic type provided with some kind of liner is most frequentlyused in the industry.

With the fourth type of the air-jet mills, the goal was the increase ofmill output by a method which did not cause the shortening of the pathof free movement of particles.

In favour of this, the volume of grinding space and the number ofnozzles has been increased, increasing thereby the output of millrelative to its unit volume, however, the efficiency of energyutilization has been decreased and the extent of wear also increased.This type of mill is known as the Jet-O-Mixer or Reductionizer.Addressed to the reduction of wear, the design of the Double-Impact-Millappeared on the market. In the return branch of the upper part of themill a so-called directional-change-sizer and further grinding nozzleshave been applied in some cases. The achievement of a grain size of 1 μmby the use of these types of mills was not realized.

Fluid bed air-jet-mills may be included in the fifth type (e.g. DE No.3140294 C2) where frequency of collision of particles and thus theefficiency of grinding is increased by the use of four nozzles of largerdiameter, when compared to the previously mentioned methods, and whichare operated opposite to each other and are located in the bottom partof a large container. The nozzles operate to fluidize the entire amountof material in the container with air flow entraining the finer grainsto be passed through a rotating sizer in the top section of thecontainer. Meanwhile, the coarser fraction slips back down the wall ofthe container for repeated grinding.

The above device possesses reasonably good grinding and sizingefficiency, however, it is not suitable for fine (below 10 μ) grinding,partly because, due to the short path length of the particles (highdensity), the resultant impact energy of the particles is small, partlybecause even the speed of rotation of the revolving part of the sizer,controlling the fineness of the end product, cannot be increased beyonda certain limit. Other disadvantages of this design lie in the conditionthat the revolving part of the sizer is exposed to high wear and, due tothe high pressure in the grinding space, charging of the material canonly be carried out by the use of an involved sluice system. Based onthe knowledge obtained from the aforementioned types of mills, it can beestablished that the efficiency of air-jet facilities is favourable onlyif the particles possess high energy and the probability of impacting isalso high, however, as the free path length required for particles tobecome accelerated shortens, the impact energy also diminishes,therefore, there is a compromise forming the basic problem in theoperation of air-jet facilities: either to increase the free path lengthand make the ground product finer along with diminishing efficiency ofthe mill, or to increase the number of impacts which while resulting ina coarser product, improves the grinding efficiency and performance ofthe mill.

OBJECTS AND SUMMARY OF THE INVENTION

The invention is drawn towards the development of an air-jet millstructure capable of fine grinding very hard, elastic and/orthermoplastic materials to below 10 μm, which is energy-saving, and doesnot contain any moving parts, thus exhibiting high resistance toabrasion. It features an inner sizer as an integrated part of the mill.The sizing control is sharp and the unit does not consume a majorportion of the comminution energy.

The invention is based on the recognition of the following:

grinding efficiency can considerably be improved by the adoption ofpregrinding, by adequately increasing the number and appropriatelyarranging the feed nozzles, by the recirculation of coarser fractionsinto the pregrinding chamber, by selecting the appropriate number andarrangement of peripheral grinding nozzles the grinding work can beperformed equally by all the nozzles,

by eliminating moving parts, and supplying the material to be ground ina horizontal plane in a tangential direction, a minimum of abrasion wearcan be achieved, and this wear can be confined to the easily replaceableelements of the pregrinding chamber,

considerable diminution of wear of the pregrinding chamber can beattained by the use of as many auxiliary nozzles as the confluentlyjetting main nozzles and which are aimed at diverting the material fromthe wall,

by the development of a new profile lining element, and by the adoptionof a curb of blades of adjustable blade angle, a very sharp inner orinternal sizing can be obtained (within the grain limits of 0.1-100 μ)without any input of energy, just by the utilization of energy leftafter grinding,

by appropriate axial adjustment of the nozzles, vacuum is generated inthe charging orifice, enabling the charging and refeeding of materialsto be ground as well as surface treating materials and/or coolants. Thisenables the apparatus to be suitable for surface treatment and also ofgrinding heat-sensitive elastic materials,

setting of the angle of the peripheral nozzles in the grinding space(diverting the material from the wall) reduces the number of impacts onthe wall, thus enabling an optimum adjustment of the movement of thematerial.

With the construction of the air-jet mill, according to the presentinvention, material feed takes place in the horizontal plane of thegrinding space in tangential directional, thus besides good grindingefficiency, wear of the grinding space, occurring with the micronizertypes, can be reduced. The use of a pregrinding chamber, resulting in asmaller diameter feed material, considerably improves the efficiency ofgrinding. The number and positioning of peripheral nozzles should beselected so as to allow each nozzle to perform an equal amount ofgrinding work. It is expedient to charge material into the grindingspace after every second nozzle.

For instance, if six peripheral nozzles are applied and threetangentially set jet pipes are used, the grinding performance can beincreased three-fold according tests made.

In order to further improve the grinding efficiency, the coarserfraction from the grinding space is returned to the pregrinding spacethrough a channel by the effect of vacuum generated in the chargingchannel (i.e. aspiration). The material charging nozzles (injectingchannels) are connected with the pregrinding chamber, the latter beingprovided with wear resistant lining, where two or more nozzles set at90°--180° angles to each other and/or shifted also in the plane, areinjecting the material confluently. In the case of running more than twoconfluent nozzles, two are performing material feed, the remaining ones,however, are decreasing wear by reducing the probability of particlesimpacting against the wall of the chamber.

The arrangement of nozzles, according to the invention, by giving riseto generation of vortices, i.e. by increasing the number of impacts ofparticles, results in very good grinding effect in the pregrindingchamber. The injecting nozzles are suitable also for introducing surfacetreating materials and/or a coolant into the system corresponding to theparticular grinding technology required. Through proper adjustment,vacuum would be generated in the feed orifice causing the coolant orpreground material to be aspirated into the grinding space. This is madepossible by an injection nozzle coaxial with the injecting pipeexhibiting the highest pressure in the system and suitable foraccelerating the preground material to an adequate velocity (themultiple of sound velocity) in spite of the vortices generated by theconfluent nozzles.

Upon investigating the relation between comminution efficiency andpressure, it has been established that efficiency improves slightly upto 9 bar pressure, then rapid increase was experienced in the 9-5 barrange and finally severe agglomeration takes place in the range 15-25bar pressure, depending on the material, deteriorating the efficiency ofcomminution.

With one potential alternative design of the air-jet mill according tothe invention, there are four nozzles connected to the pregrindingchamber in tangential direction. The flow proceeds perpendicular to themain grinding space and the gas jets are generating the vortex bycontacting a circle of comparatively small radius. With this solution,two nearly horizontal nozzles are shooting together the material to beground, the other two nearly vertical nozzles, however, are conductinggas or air to the system. The latter may be linked to the containers ofreagents or coolant. The injecting tubes are connected tangentially atthree points to the grinding space where the six peripheral nozzlesrotatable around their vertical axes are located symmetrically. Thegrinding chamber is also connected to the pregrinder by materialsrecirculating pipes in order to return the coarse fraction forpregrinding. The inner sizer is designed to be symmetrical with the axisof the grinding space, said sizer consisting of a surface area of ahyperboloid of revolution and an adjustable curb of blades having thesame axis as the discharge stub for the ground product.

The other potential alternative design of the air-jet mill according tothe invention differs from the one outlined above in the mode ofconstruction of the pregrinding chamber. The pregrinder has twoconfluent shooting nozzles set at an angle within the range of 150°-180°and another injecting nozzle placed in the axis of the blow-pipe. Allthree may be connected with a charging funnel each.

With either design of the air-jet mill, the material to be ground flowsin the required quantity by gravity from the storage container onto thecharging dish/disc feeder, the latter being eccentrically shaken. Auniform stream of material is supplied from the disc feeder into thematerial charging funnels located along the edge of the dish.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in a couple of embodiments in the drawingsattached, where:

FIG. 1 shows the cross section of a possible construction of theinvention.

FIG. 2 shows a section taken along the line I--I indicated in FIG. 1.

FIG. 3 shows a cross section of a second possible construction.

FIG. 4 shows a longitude section of the material charging system of theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one possible embodiment shown as an example in FIGS. 1 and 2, threepregrinding chambers (1) are connected to the grinding space (2). Theconfluent nozzles (7) located in the nozzle casing (6) and the injectingnozzles (8) are linked to the pregrinding space (5) which is equippedwith a wear resistant lining, by means of charging ducts (9) the latterbeing realized by Laval-profiles. There are also vertical auxiliarynozzles (8a) provided for injecting cooling medium of reagent forsurface treatment of the particles. The pregrinder is connected with thegrinding space via a blowing duct (3) and a material return duct (4).The peripheral grinding nozzles (10) are located symmetrically in thegrinding space (2) and can be swivelled in the horizontal plane byturning the angle setter (11). In the grinding space (2) there is a wearresistant lining (12) and an adjustable curb of blades (13) with anassociated gear rim (15) and a stub (14) for setting the angle ofblades. The curb of blades is thus an adjustable sizer effecting controlover the size of the product discharge. As it can be seen from thedrawings, the discharge stub (17) is located in the axis of the casing(16) of the air-jet mill. Material charging stubs (18) and air inletstubs (19) are also provided on the mill.

FIG. 3 shows another possible embodiment. In this case, the pregrindingchamber is of a simpler design; the axes of confluent nozzles (7)located in the nozzles casings (6) are set at 150°--180°, preferably150° angles to each other. The angles to be set for the confluentnozzles should be selected as a function of the radius of thepregrinding space and in a way that is related to of the velocity ofpreground material pointing to the grinding space. The development ofthe grinding casing (2), the peripheral grinding nozzles (10) and theadjustable curb of blades (13) as well as charging of material isidentical with those outlined above. Material charging system isdescribed with reference to FIG. 4. The material storing hopper (20) isequipped with adjustable louvres (21). The material flows from thehopper onto disc feeder (23) which is shaken by an eccentricallyoperating unit (22) spreading the material uniformly and distributingthe same into charging funnels (24) the latter being connected to thematerial supply stub (18).

The main advantage of the air-jet mill of the invention is that, incontrast to the mills known so far, the invention is capable ofproducing grain fractions less than 10 μm in size, capable of cryogenicgrinding of thermoplastic materials and applying surface treatingmaterials contemporarily with grinding. A further advantage of thefacility lies in the excellent utilization of energy achieved throughthe novel shaping of the inner sizer (i.e., the adjustable curb ofblades). The efficient utilization of the grinding energy, when ascompared to a conventional device, is increased by up to fifty percent.Particular advantage lies in that the unit does not comprise any movablepart which could be exposed to severe wear and the only oneconstructional part, the lining of the pregrinder which is exposed tothe greatest wear can easily and be replaced at little expense.

We claim:
 1. An air-jet mill for fine grinding and sizing, comprising atleast one pregrinding chamber, a grinding chamber defining acircular-shaped grinding space connected by a tangential blow pipeoriented to introduce particulate material into said grinding spacethrough an injecting nozzle in a direction tangential to the peripheryof said grinding space at a point on said periphery and with anorientation which causes air flow in a first direction,peripheralgrinding nozzles opening into the grinding space and a discharge stubfor the ground material, a sizer positioned in the grinding chamber,said pregrinding chamber being connected with the grinding space by saidtangential blow pipe and by a material return channel, said peripheralgrinding nozzles opening into the grinding space arranged symmetricallyalong a circle and the number of said peripheral grinding nozzles beingtwice the number of injecting nozzles.
 2. Air-jet mill according toclaim 1, wherein said sizer comprises a curb of blades located in thegrinding space and mounted in a replaceable fashion and wherein theangle of blades is adjustable during operation.
 3. Air-jet millaccording to claim 1 wherein three or more confluent nozzles andauxiliary nozzles are set preferably at 90°--180° angles to each otherand each located in a nozzle casing and an injecting nozzle is locatedin the axis of the blow pipe, said confluent, auxiliary and injectingnozzles being connected to the pregrinding chamber and being adjustable,in the axial direction.
 4. Air-jet mill according to claim 3, whereinhorizontal nozzles of the pregrinding chamber are connected with amaterial supply stub, the auxiliary nozzles being connected to thestorage tanks of the coolant and/or surface treating material,respectively.
 5. Air-jet mill according to claim 1 wherein saidperipheral grinding nozzles are replaceable and rotatable in thehorizontal plane.
 6. Air-jet mill according to claim 1, wherein thegrinding space and the pregrinding space are provided with a very hardreplaceable lining made preferably of sintered corundum or variouscarbides or glass-hard hardened steel.
 7. Air-jet mill according to 1wherein a connection with a material storing hopper is accomplished byadjustable louvres, a disc feeder shaken by an eccentrically operatedunit, and further including charging funnels.
 8. An Air-jet millaccording to 1, wherein the grinding chamber has an interior surfaceshaped such that it rises along a circular arc on the bottom andexhibits a hyperboloid on the top.
 9. An air-jet mill for fine grindingand sizing, comprising three pregrinding chambers, a grinding chamberdefining a circular-shaped grinding space connected by tangential blowpipes, oriented to introduce particulate matter in a directiontangential to the periphery of said grinding space at a point on saidperiphery, peripheral grinding nozzles opening into the grinding spaceand a discharge stub for the ground material, a sizer positioned in thegrinding chamber, said pregrinding chambers being connected with thegrinding space by one each of said tangential blow pipes and by materialreturn channels said peripheral grinding nozzles opening into thegrinding space are arranged symmetrically along a circle.
 10. An air-jetmill for fine grinding, surface treatment and/or cooled grinding,comprising:(a) at least one pregrinding chamber; (b) an annular grindingchamber connected by a flow passage to each pregrinding chamber; (c) atleast three injection nozzles communicating with the flow passagethrough each pregrinding chamber; (d) peripheral grinding nozzlescommunicating with the grinding chamber, said peripheral grindingnozzles being arranged symmetrically; (e) a discharge passage for theground product extending axially from said grinding chamber; (f) aninner sizer; and (g) a material return passage between the grindingchamber and each pregrinding chamber.
 11. An air-jet mill according toclaim 10, wherein each pregrinding chamber is connected to the grindingchamber by a plurality of flow passages extending tangentially to thegrinding chamber.
 12. An air-jet mill according to claim 10, wherein thepregrinding chamber is one of a plurality of pregrinding chambers eachhaving an injection nozzle and the number of the peripheral nozzles istwice the number of the injection nozzles.
 13. An air-jet mill accordingto claim 10, wherein an annular array of blades is disposed in thegrinding chamber, said blades being replaceable, the grinding chamberhaving a surface shaped such that it rises along a circular arc on thebottom and exhibits a hyperboloid on the top.
 14. An air-jet millaccording to claim 10, wherein the pregrinding chamber is provided withthree or more confluent nozzles located in a nozzle casing, a materialinjection nozzle being located in the axis of the flow passage.
 15. Anair-jet mill according to claim 14, wherein said confluent nozzles areset at an angle of 90°-180° to each other.
 16. An air-jet mill accordingto claim 10, wherein each of said injection nozzles is adjustableaxially thereof.
 17. An air-jet mill according to claim 10, wherein theperipheral nozzles are replaceable and rotatable in a horizontal plane.18. An air-jet mill according to claim 10, wherein the grinding chamberand the pregrinding space are each provided with a very hard replaceablelining.
 19. An air-jet mill according to claim 18, wherein the liningsare made of sintered corundum or various carbides or glass-hard hardenedsteel.
 20. An air-jet mill according to claim 10, and including amaterial storing hopper connected to said mill by adjustable louvres, adisc feeder,a vibrator for vibrating the disc feeder and charging funnelmeans for funnelling the material from the disc feeder into thepregrinder chamber.
 21. An air-jet mill according to claim 10, whereinthe injection nozzles of the pregrinding chamber comprise horizontalnozzles connected with a material supply passage and vertical nozzlesconnected with storage chambers for coolant and/or surface treatmentmaterial.